[LTO] Teach computeLTOCacheKey to return std::string (NFC) (#105331)

Without this patch, computeLTOCacheKey computes SHA1, creates its
hexadecimal representation with toHex, which returns std::string, and
then copies it to an output parameter of type SmallString.

This patch removes the redirection and teaches computeLTOCacheKey to
directly return std::string computed by toHex.  With the move
semantics, no buffer copy should be involved.

While I am at it, this patch adds a Twine to concatenate two strings.

llvm/include/llvm/LTO/LTO.h

@@ -62,11 +62,9 @@ void thinLTOInternalizeAndPromoteInIndex(
 
 /// Computes a unique hash for the Module considering the current list of
 /// export/import and other global analysis results.
-/// The hash is produced in \p Key.
-void computeLTOCacheKey(
-    SmallString<40> &Key, const lto::Config &Conf,
-    const ModuleSummaryIndex &Index, StringRef ModuleID,
-    const FunctionImporter::ImportMapTy &ImportList,
+std::string computeLTOCacheKey(
+    const lto::Config &Conf, const ModuleSummaryIndex &Index,
+    StringRef ModuleID, const FunctionImporter::ImportMapTy &ImportList,
     const FunctionImporter::ExportSetTy &ExportList,
     const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
     const GVSummaryMapTy &DefinedGlobals,

llvm/lib/LTO/LTO.cpp

@@ -88,10 +88,10 @@ extern cl::opt<bool> EnableMemProfContextDisambiguation;
 
 // Computes a unique hash for the Module considering the current list of
 // export/import and other global analysis results.
-// The hash is produced in \p Key.
-void llvm::computeLTOCacheKey(
-    SmallString<40> &Key, const Config &Conf, const ModuleSummaryIndex &Index,
-    StringRef ModuleID, const FunctionImporter::ImportMapTy &ImportList,
+// Returns the hash in its hexadecimal representation.
+std::string llvm::computeLTOCacheKey(
+    const Config &Conf, const ModuleSummaryIndex &Index, StringRef ModuleID,
+    const FunctionImporter::ImportMapTy &ImportList,
     const FunctionImporter::ExportSetTy &ExportList,
     const std::map<GlobalValue::GUID, GlobalValue::LinkageTypes> &ResolvedODR,
     const GVSummaryMapTy &DefinedGlobals,
@@ -355,7 +355,7 @@ void llvm::computeLTOCacheKey(
     }
   }
 
-  Key = toHex(Hasher.result());
+  return toHex(Hasher.result());
 }
 
 static void thinLTOResolvePrevailingGUID(
@@ -1488,11 +1488,10 @@ class InProcessThinBackend : public ThinBackendProc {
       // no module hash.
       return RunThinBackend(AddStream);
 
-    SmallString<40> Key;
     // The module may be cached, this helps handling it.
-    computeLTOCacheKey(Key, Conf, CombinedIndex, ModuleID, ImportList,
-                       ExportList, ResolvedODR, DefinedGlobals, CfiFunctionDefs,
-                       CfiFunctionDecls);
+    std::string Key = computeLTOCacheKey(
+        Conf, CombinedIndex, ModuleID, ImportList, ExportList, ResolvedODR,
+        DefinedGlobals, CfiFunctionDefs, CfiFunctionDecls);
     Expected<AddStreamFn> CacheAddStreamOrErr = Cache(Task, Key, ModuleID);
     if (Error Err = CacheAddStreamOrErr.takeError())
       return Err;

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

@@ -382,13 +382,13 @@ class ModuleCacheEntry {
     Conf.RelocModel = TMBuilder.RelocModel;
     Conf.CGOptLevel = TMBuilder.CGOptLevel;
     Conf.Freestanding = Freestanding;
-    SmallString<40> Key;
-    computeLTOCacheKey(Key, Conf, Index, ModuleID, ImportList, ExportList,
-                       ResolvedODR, DefinedGVSummaries);
+    std::string Key =
+        computeLTOCacheKey(Conf, Index, ModuleID, ImportList, ExportList,
+                           ResolvedODR, DefinedGVSummaries);
 
     // This choice of file name allows the cache to be pruned (see pruneCache()
     // in include/llvm/Support/CachePruning.h).
-    sys::path::append(EntryPath, CachePath, "llvmcache-" + Key);
+    sys::path::append(EntryPath, CachePath, Twine("llvmcache-", Key));
   }
 
   // Access the path to this entry in the cache.